Such aircraft towing vehicles are disclosed, for example, in German patent publications DE-OS 33 27 628 and DE-OS 33 27 629. This kind of towing vehicle forms an essentially rigid unit with the clamped nose wheel of the plane being towed so that even very large and heavy planes can be towed and maneuvered more simply, safely and rapidly than was possible with towing vehicles which are connected to the nose wheel landing gear ("nose gear") with a draw bar. It becomes therefore possible to use towing vehicles of the kind to which the present invention relates for taxiing fully loaded aircraft between the terminal and the runway. This eliminates the need to use the aircraft engines to taxi the plane to and from the runway which results in a significant savings in fuel and other costs and greatly abates the often objectionable noise around airports. This cannot be accomplished with towing vehicles employing draw bars because the instability in the towing unit, i.e., the towing vehicle and the aircraft connected thereto, permits only very low towing speeds.
The concept of a towing vehicle that is rigidly connected to the nose wheel and the contemplated relatively high towing speeds present additional demands for the towing operation itself. One requirement is that during towing the forces that are transferred to the nose wheel may not overload or damage the nose gear. Another requirement is that under no circumstances may the connection between towing vehicle and nose wheel become uncoupled during towing. This contrasts with conventional towing vehicles employing draw bars in which a shear pin forms the connection between the nose wheel and the vehicle. It will shear off under overload conditions and thereby release the nose wheel before it could be damaged by excessiVe forces.
Forces which can cause directional instability of the towing unit and overload conditions in the nose gear occur primarily during braking. If, when the brakes are applied, the longitudinal axis of the towing vehicle is at an angle to the longitudinal axis of the aircraft, as occurs, for example, during turning, then the ground frictional forces of the vehicle's wheels, on the one hand, and the momentum of the large aircraft mass on the other, generate a rotational moment about a vertical axis of the vehicle. It tends to increase the angle between the axes of the vehicle and the aircraft and can result in jack-knifing during which the towing vehicle spins out under the aircraft and twists the nose wheel relative to the aircraft. An excessive twisting of the nose wheel can seriously damage the entire nose gear.
Maneuvering towing vehicles of the kind proposed herein is, to be sure, inherently more directionally stable and less prone to jack-knifing than maneuvering vehicles of the draw bar type. Nevertheless, an unstable condition may arise during braking if the rear wheels prematurely lose ground traction, which can result in an excessive nose wheel rotation and, consequently, in damage to the nose gear.
U.S. Pat. No. 4,113,041 discloses a towing vehicle which is connected to the wheel landing gear via a draw bar. To avoid unstable driving conditions and, in particular, jack-knifing, the drive and/or braking power of the towing vehicle are controlled and limited as a function of the pulling or pushing force in the draw bar. Braking is primarily effected with the aircraft's landing gear brakes. They are either operated directly from the towing vehicle via remote control or by the aircraft's pilot with the aid of braking commands emanating from the towing vehicle or visually signaled him. Such a system either requires a compatibly equipped aircraft having a braking system which can be remote controlled from the towing vehicle itself or, should the pilot have to rely on braking commands relayed to him from the towing vehicle, it will be extremely unreliable and failure prone.
It is axiomatic that a new type of towing vehicle, e.g., the one of the present invention, will only be employed if it can be used to tow all commonly encountered passenger airliners without requiring their modification with expensive additional equipment to make them compatible with the new towing vehicle. Furthermore, it is desirable that operation of the towing vehicle be either completely independent of, or as little dependent as possible, on complicated, preprogrammed controls, associated sensors. signal transmitters and transmission lines, and the like. Because of the rugged working conditions typically encountered at airports, extreme weather conditions, and electronic interference resulting from the proximity of powerful sources of electromagnetic signals, etc., such controls are very susceptible to malfunction. A malfunctioning control system for the towing vehicle can create dangerous situations which, for safety reasons, are intolerable.
It is basic knowledge (cf.. for example. DE-OS 33 11 556) that, in braking systems for standard vehicles, front and rear wheel braking forces are differentially controlled in such a way that the braking force generated by the front wheels is always greater than that generated by the rear wheels. This compensates for the relatively greater forces to which the front wheels are subjected during braking. DE-OS 28 15 337 discloses a braking system with dual brake circuits in which the brake circuits can include various combinations of front and rear wheels. For example, FIG. 4 discloses an arrangement in which each circuit encompasses both front wheels and one of the rear wheels. However, the publication is silent as to the actual braking forces that are to be applied to the wheels controlled by the respective brake circuit.